Building Cars Powered by Hot Air

Sir Isaac Newton.  That’s where this project began. Sir Isaac Newton and his three Laws of Motion.

Law Number One:  An object at rest tends to stay at rest.  An object in motion tends to stay in motion.  These conditions cannot change without being acted on by a force.

Law Number Two:  Force equals mass times acceleration.  The more force, the more acceleration.

Law Number Three:  For every action there is an equal and opposite reaction.

We talked about these laws and were helped with our understanding of them by watching this Youtube video:

The students, in groups of two, were about to build cars.  The body and wheels were cut from a styrofoam meat tray.  The axle, to which the wheels were held steady with clay, was a wooden stir stick.  The axle was positioned in a straw which was taped to the bottom of the car body.  The car was powered by a balloon.  All of the materials came as part of a kit that I purchased from Carolina Biological .  What I liked about having this as a kit is that everyone had the exact same materials and the exact same set of directions.  Each pair of students had to read and follow around 30 directions in order to complete their car.

The engagement and cooperation within the groups was impressive.  I had them read through the directions with their partner before coming to get the materials.  I wanted them to have an idea of where this was going, and what the materials were for.  Once they had the materials, they read aloud the directions carefully and began assembling their car.

On the second day of class, students were fine tuning.  Once the car was ready for testing, the students went into the hall outside our room to make sure the wheels were steady and the car moved straight.  There were quite a few cars that veered to one side or another.  In that case the students took the car back in to make adjustments to the wheels.  When the cars were “competition ready”, we went down to the cafeteria to race them.  Here is video of that for each of my three classes.

Two of the cars from the third class went extraordinary distances.  The winner went 331 inches (27 1/2 feet)!  The second highest distance was 318 inches (26 1/2 feet).  No other car all day went even half that distance!  At this point there was so much to talk about!

Why did some of the cars not move at all?
Why did the wheels keep falling off?
How were the winning cars different from the others?

The first thing we did back in the classroom was interview the builders of the two winning cars.  The four students involved gave a lot of credit to the wheels of their cars.  They spent time making sure they were uniformly round.  They sanded them to help the car roll smoothly.  And they measured to make sure the axle was as close to the center of the wheel as possible.  Then they used the clay to make them snug on the axle.  No wobbling!

Next we were ready to review Newton’s Laws of Motion.

We considered the second law: Force equals Mass times Acceleration.  The balloon was the force that powered the car.  Could we alter that? Would it help?  Would more balloons result in more force?  Will several balloons of different sizes lose all their air at the same rate?   What would happen if the mass was increased?  How would that impact the speed or distance?  What if it was decreased?  What could we make the body and wheels from besides styrofoam?

We considered the first law:  An object in motion tends to stay in motion and an object at rest tends to stay at rest unless acted upon by a force.  What force caused the cars to slow down and stop?  Was it just the lack of air in the balloon?  What about friction?  What would happen if we altered the wheels?  What else could we make them out of?  What if we varied their width?  How important is it to cut them so they are perfectly round?  How important is it to measure to find the exact center of the wheel when attaching it to the axle?

We considered the third law:  For every action there is an equal and opposite reaction.  The air from the balloon is being released in one direction, but the car is moving in the opposite direction.  Does the position of the balloon matter?  Does the angle of the balloon and straw matter?  Does the order of the different sized balloons matter?

Equipped with the experience of having already built one car along with the understanding gained from discussing the Laws of Motion, the same groups were asked to build another car.  This time they could use whatever they wanted.  The only thing I discouraged was bringing manufactured wheels off of a toy car.  I put out cardboard, more balloons of different sizes, cardboard tubes and the remaining supplies that came with the initial kit.  The students got started, knowing that they would have the opportunity to bring additional supplies from home.

On day four of this activity, the students had some time to get their cars “competition ready”.  We had a car show (it seemed necessary now that no two cars looked alike).  It was so interesting to see the variations.  During work time, those who sought to use five balloons realized that it was difficult to keep the air in three while trying to blow up the others.  In the end, three balloons was the most that anyone used.  Here are the cars and then races from the three classes:

Even when the cars didn’t leave the starting line, the students laughed and enjoyed this challenge!  In the end, the group whose first car went the furthest, built a second car that also went the furthest!  Their second car was quite different from their first car in that the wheels in the second car were Kerr jar lids!  They struggled during work time in keeping them from wobbling, but by the time the race was run, they had figured that out!

Another group used plastic bottle caps with holes drilled for the axle.  The wheels worked beautifully, but this group struggled with attaching their balloon.  Two other interesting sets of wheels were made out of cardboard tubes.  One group used rubber bands around the tubes to grip the floor!  These two groups struggled with attaching the wheels securely to an axle.

At the beginning of class the next day, I asked the students to share what they had learned while doing this project:

  1.  The cardboard wheels went faster than the styrofoam wheels.
  2.  The wheels on each axle needed to be the same size.
  3.  The wheels needed to be sanded smooth.
  4.  It really helped having a partner to talk with and to help hold parts while taping.
  5.  Having more balloons didn’t always work.  It was difficult to inflate and release several balloons all at once.  Sometimes the first balloon was leaking air as the second was being filled.
  6.  It gets too crowded to have too many balloons.
  7.  The car body needs to be big enough to keep the balloons off the floor.
  8.  Large balloons worked better than small ones.
  9.  Masking tape worked well to tape the wheels to the axle.

When you watch the races of both car 1 and car 2, it is pretty obvious that overall there was more success with car 1.  But in the end that didn’t matter.  It wasn’t the end product that was the most important thing here.  It was the mission and the process.  It was the student input, the focus and the cooperation.  Everyone had moments of frustration, but they worked through those moments.  Students cheered each other on and made this a memorable fun activity.  When asked if I should repeat this activity next year, ALL students said, “YES!”

An Opportunity to Find Out What Works and What Doesn’t

“Your mission, ladies and gentlemen, is to make a balloon travel along a string.  Once you are satisfied that you have successfully accomplished that, you are to adjust your design to make the balloon travel faster.  In the end I would like you to see just how fast you can get the balloon to travel to its destination at the end of the string.”

Those were the instructions.  The materials each team of two started with were a balloon, a straw, and whatever length of string they wanted.  If they wanted to use additional materials, they had to ask.  I said yes to all requests that did not present safety concerns.  And they were off!

This was such a fascinating process to watch.  Most immediately began blowing up the balloons and tying them off – but then what?  Why were they given a straw?  “Do we have to use the straw?  How long should the string be?”

“Yes, use the straw.  Cut the string where you think it should be cut.”

There was that slight hesitation.  Those moments of letting the idea sink in that I wasn’t going to give them step by step directions.  But quickly that hesitation turned to excitement and concentration on the task.  I stepped back at this point and became the observer and recorder of the event.  I did not blow up balloons, and I did not get drawn into any group’s brainstorm.  I was eager to watch how each group would work this out.

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At least two groups tried to use what they knew about balloons.  They rubbed the balloon in their hair to create static electricity.  They were disappointed to see that it wasn’t enough to keep the balloon sticking to the straw.



They tried seeing if the static electricity they were creating could be strong enough to pull the balloon along the string.  At this point the balloon was taped to a straw through which the string was threaded.   Then the balloon was rubbed in hair.  The girl followed the balloon as it was released on the string,  hoping her charged hair would pull the balloon.  This worked, but it was not speedy.  They abandoned the idea of using static electricity in this process, although other groups were curious by what this group was doing, and I saw them trying things with it as well.

Most everyone knew that by having one end of the string higher than the other, gravity would help that balloon move along the string.  There was one group, however, that created a two person game.  They rigged the strings in such a way that each person held the end of two strings.  As the first person pulled one string back, the balloon moved toward the other person.  Then the second person pulled one string back, and the balloon traveled back to the first person!  They added to the fun of their new game by drawing a face on their balloon.  What an unexpected invention!

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Those who were taping one end of their string to the wall quickly learned that masking tape sticks better than scotch tape!  I did not let anyone attach their string to the ceiling, so they reached up along the wall as high as they could reach.  It was interesting to see the groups experiment with the angle of descent.  They learned that it indeed made a difference!

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While some were learning that the angle of descent was important, others were learning that the tautness of the string was important.  A few trials in which the balloon slowed and stopped along the way down, made the members of those groups tighten up the string.  One group even rubbed the string with closed markers, hoping to make the straw move more smoothly.

The next interesting thing I saw happening was weights being added.  This came in different ways.  Some added the weight by taping it directly to the balloon.  Others taped it to the straw.  Sometimes the weights were added in random places on the balloon and sometimes the weights were equal on either side of the balloon.  There was so much experimentation going on!  And as I had hoped, trying out each great idea always seemed to inspire another!

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It was interesting to note that some models had the balloon traveling above the string and some had it hanging below the string.  It appeared that the faster model had the balloon above the string and the weights attached to the straw.  One group used the cardboard tube from gift wrap and taped baggies full of Jenga blocks to it.  That balloon went really fast, but the baggies which were taped to the tube with duct tape kept falling off upon impact.

Another innovative idea was to tie two strings side by side.  The straw was cut in half and the strings were threaded through each piece.  The balloon was then taped to the two straws and set on its descent.  I loved that they thought of it and tried it.  In the end they learned that using two strings slowed the balloon down rather than to speed it up.


Now if you are like me, you’ve been wondering when someone would think to blow up a balloon but NOT tie it off.  Instead, hold it shut while it gets taped to the straw.  Then let go and watch the balloon power itself!  Funny, but only five out of the thirty groups that experimented throughout the day played around with this idea.  One of the groups that used the untied balloon as an “engine” combined it with other great ideas.  They had a tied off balloon taped to the bottom of the straw with weights (markers and glue sticks) taped to the straw.  They blew up a second balloon and taped it to the top of the straw just before launching.  After a few successful descents, they dressed up their model with airplane-type wings and called it the U.S.S. Static Electricity!

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I think I enjoyed this 45 minute activity as much as the students.  They were never done trying out different ideas.  There was that one group that in the first five minutes said, “We can’t get it to go.  We can’t do this.”  But given five more minutes, they were busy, busy, busy.

After clean up, I gathered everyone together and asked what they had learned.  You see, the point of this was never to have the fastest balloon in the class.  The point was to keep modifying or trying different ideas and to improve the original design several times.  To that end, everyone achieved success!